One method for the direct reduction of iron ore by means of a reducing gas is based on reducing finely particulate iron ore having a grain size of 0.005 to 12 mm in a fluidized bed. The fluidized bed is obtained by injecting reducing gas into the finely particulate iron ore in a fluidized bed reactor in a fluidized bed reduction system. The finely particulate iron ore is held in suspension by the stream of gas and reacts with the reducing gas, in which case it itself is reduced and the reducing gas is oxidized. After a certain residence time in the fluidized bed reactor, the thus reduced finely particulate material is removed. It is known to reduce the finely particulate iron ore in a cascade of a plurality of fluidized bed reactors by reducing gas. By way of example, in the FINEX® process or in the FINMET® process, the finely particulate iron ore is conducted in countercurrent to a stream of reducing gas through a cascade of a plurality of fluidized bed reactors.
The material which is removed from the last fluidized bed reactor, as seen in the direction of flow of the finely particulate iron ore, and which is largely reduced is usually subjected to a final reduction step or melt-down step for producing pig iron. This material is also referred to as finely particulate direct reduced iron (DRI). Such a final reduction step or melt-down step is carried out in a melter gasifier, for example. In such a melter gasifier, a reducing gas is produced from carbon carriers and oxygen by gasification reactions, pre-reduced iron carriers—for example precisely the largely reduced material DRI removed from the last fluidized bed reactor—are subjected to a final reduction and also the pig iron produced in the process is melted down. The final reduction step or melt-down step can also take place, however, in a type of melt reduction system which differs from a melter gasifier, or for example in a blast furnace.
The DRI can also be used, however, as an iron carrier for another application, for example for steelworks for instance in arc furnaces or converters.
To ensure an efficient procedure, iron carriers intended for use in a melter gasifier should have a grain size distribution which avoids firstly the negative effects associated with excessively small grain sizes of the iron carriers—such as nonuniform gas distribution in the melter gasifier—and secondly the negative effects on the melter gasifier operation associated with excessively large grain sizes of the iron carriers—such as delayed melt-down behavior and increased proportion of direct reduction, and as a consequence thereof also higher levels of reducing agent consumption. In order for it to be possible to use the finely particulate material DRI removed from the last fluidized bed reactor in the melter gasifier as an iron carrier, it is processed, for example by compacting, to form pressed articles. To this end, the material DRI removed from the last fluidized bed reactor is initially fed to a collection tank, also referred to as a DRI fines bunker, and from there is fed to a compacting system. Since the DRI is obtained in finely particulate form, use is made of the term DRI fines bunker within the context of the present application, where the part fines is present in this term on account of the finely particulate size of the DRI. The collection tank—or DRI fines bunker—is required during operation in order to be able to offset brief system disruptions, which can occur upon pneumatic conveying between the last fluidized bed reactor and the collection tank—or DRI fines bunker; the collection tank—or DRI fines bunker—acts in this case as a buffer store for the material feed of DRI in system parts arranged downstream thereof.
When DRI is used as an iron carrier in a blast furnace or steelworks, too, the DRI is compacted.
In fluidized bed reduction processes, spent reducing gas, i.e. reducing gas used for the reduction reaction, is generally removed from the last fluidized bed reactor, as seen in the direction of flow thereof, and discharged as so-called off-gas. Since it has indeed passed through one or more fluidized bed reactors with finely particulate iron ore, the off-gas entrains finely particulate material containing, inter alia, finely particulate iron oxide from the iron ore and also finely particulate iron formed during the reduction and some carbon. To separate this dust load, the off-gas is dedusted, for example by means of a dry dedusting apparatus, for instance a filter apparatus, by means of bag filters or ceramic filters or by means of a cyclone. The separated material, which in the case of dry dedusting is a dry dust containing iron oxide, has—particularly since it indeed contains not only iron oxide but also material already reduced to form iron—a high iron content and carbon content, and therefore for economical reasons should be used as an iron carrier raw material—for example in pig iron production in a melter gasifier or blast furnace, or in steelworks. It should be used preferably in pig iron production processes associated with the fluidized bed reactors in which it was formed. However, since the separated material is significantly finer than the finely particulate material fed to the fluidized bed reactors, and it is even too fine to be added, for example, to the melter gasifier, its economic use provides difficulties.
If a fluidized bed reduction system is shut down, the fluidized bed reactors have to be emptied in order to avoid passage of the material present therein through the distributor bases and agglomeration and also clumping. Just as in the dry dedusting apparatuses for off-gas, finely particulate material is obtained in this case, containing, inter alia, finely particulate iron oxide from the iron ore and also finely particulate iron formed during the reduction and carbon. This material, too, should be supplied for use as an iron carrier raw material, for example in pig iron production in a melter gasifier or blast furnace, or in steelworks.
The use of such finely particulate materials by addition into the fluidized bed reactors is not possible, since the finely particulate materials would to a large extent be blown immediately back out of the fluidized beds, because in terms of size they lie predominantly below or in the region of the separation size of the cyclone present in the fluidized bed reactors.
The use of the finely particulate materials in dust burners for introduction into a melter gasifier is unfavorable on account of the excessively small carbon content for dust burners, because in this respect it would be necessary to use additional energy carriers, for example in the form of coal or fuel gas.